The art of surveying involves the determination of unknown positions or setting out of known coordinates using angle and distance measurements taken from one or more known positions. In order to make these measurements a surveying device frequently used is a total station. The device is generally operated by two users, one user pointing the total station at a target held by a second user.
The user traditionally makes the pointing optically, using a telescope. Robotic total stations have been developed which assist the user in locating the target and aligning to it. The robotic total stations include servomotors that allow the apparatus to be rotated to automatically align the station with the target. The automatic alignment can be done either against the reflector used for distance measurement, in which case a separate light (IR or visible) beam is sent from the robotic total station and reflected back from the reflector, or the target may be equipped with a light source.
In both cases the total station is equipped with an optical receiver to receive the alignment signal reflected from the reflector or transmitted from the light source, to be processed and used to automatically align the total station against the target. A system using both possibilities is disclosed in EP 0 465 584.
These robotic total stations can automatically find a target, lock to it and follow the target if it is moved. There are a number of different methods used to enable the total station to find a target. Normally these methods include scanning a certain “window” or angle, and locking on any target within the window.
With this type of more complex total station the user may be only one person and he is then normally working at the target in order to choose the point to be measured or mark the point to be set out. By giving commands via a radio link the user can make the station search for the target and lock to it. When the user once has marked all points to be marked on the site which are accessible for the total station, the total station has to be moved to a new location in order to measure further points on the site.
To move the total station requires not only the physical movement of the apparatus, but also, every time the station is moved, the exact location and orientation of the station has to be determined anew. The measurements have to be exact in order to correlate the new measurements to the earlier by establishing the location and the orientation of the station in relation to known reference points. Understandably this takes time, typically 30 minutes or more, and the users dependent on the total station for staking out will then have to wait, not only for the station to be moved, but also for the location and direction to be established in the new position.
This need of relocation of the total station arises on large sites to be surveyed for different reasons, e.g. the direct line of sight is obstructed by a building or a mound, or the like, or when doing surveying work for constructions of roads or runways etc. where the work site covers extensive stretches of land.
However on large building sites, more than one measuring crew may be working in the same area. This means there is a risk of a total station locking on the wrong target.
One way of aiding a total station to quickly find a target (e.g., reduce scanning time or need for scanning) is described in U.S. Pat. No. 6,035,254 wherein a surveying system using a navigation GPS receiver at the target for determination of the coarse position of the target is described. The target thereafter transmits to a total station position information estimating location of the target. The total station is then aligned coarsely with the target and can lock on it.
The prior art does not address the problem of selecting a specific target among several possible targets, when the targets appear to be closely located as viewed from the total station. A further problem is encountered when several targets and several total stations are located on the same general site; the problem of unique identification of all elements in the survey for proper linking of target with total station is further compounded when there are different users in the same general site.